Fabricating tft having fluorocarbon-containing layer

ABSTRACT

A process for fabricating a thin film transistor comprising: (a) forming a gate dielectric; (b) forming a layer including a substance comprising a fluorocarbon structure; and (c) forming a semiconductor layer including a thiophene compound comprising one or more substituted thiophene units, one or more unsubstituted thiophene units, and optionally one or more divalent linkages, wherein the layer contacts the gate dielectric and is disposed between the semiconductor layer and the gate dielectric.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional application of U.S. application Ser.No. 11/276,694, filed Mar. 10, 2006, from which priority is claimed, thedisclosure of which is totally incorporated herein by reference.

U.S. application Ser. No. 11/276,694 claims the benefit of U.S.Provisional Application No. 60/666,997, filed Mar. 31, 2005, thedisclosure of which is totally incorporated herein by reference.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

This invention was made with United States Government support underCooperative Agreement No. 70NANBOH3033 awarded by the National Instituteof Standards and Technology (NIST). The United States Government hascertain rights in the invention.

BACKGROUND OF THE INVENTION

There are a number of approaches to improving the performance of organicthin film transistors (“TFT”). One approach is to chemically modify thesurface of the gate dielectric prior to deposition of the semiconductorlayer as described in a number of the documents listed below. Furtherimprovements in performance are needed to promote the use of organicTFTs in the marketplace. In response to the need for enhanced TFTperformance, the present inventors have developed an improved organicTFT and a process for fabricating the improved organic TFT.

The following documents provide background information:

Ong et al., U.S. Pat. No. 6,770,904 B2.

Ong et al., U.S. Pat. No. 6,855,951 B2.

Kelley et al., US Patent Application Publication 2003/0102471 A1.

Kelley et al., US Patent Application Publication 2003/0102472 A1.

Kelley et al., U.S. Pat. No. 6,433,359 B1.

F. Garnier et al., “All-Polymer Field-Effect Transistor Realized byPrinting Techniques,” Science, Vol. 265, pp. 1684-1686 (Sep. 16, 1994).

A. Salleo et al., “Polymer thin-film transistors with chemicallymodified dielectric interfaces,” Applied Physics Letters, Vol. 81, No.23, pp. 4383-4385 (Dec. 2, 2002).

Laura Kosbar et al., “The effect of surface preparation on the structureand electrical transport in an organic semiconductor,” Mat. Res. Soc.Symp. Proc., Vol. 665, pp. 401-406 (2001).

S. Kobayashi et al., “Control of carrier density by self-assembledmonolayers in organic field-effect transistors,” Nature Materials, pp.317-322 and 2 pages of Supplemental Information (published online Apr.4, 2004).

Janos Veres et al., “Gate insulators in organic field-effecttransistors,” Chem. Mater. Vol. 16, pp. 4543-4555 (published on web Sep.11, 2004).

Y. Y. Lin et al., “Pentacene-Based Organic Thin-Film Transistors,” IEEETransactions on Electron Devices, Vol. 44, pp. 1325-1331 (1997).

H. Sirringhaus et al., “Integrated Optoelectronic Devices Based onConjugated Polymers,” Science, Vol. 280, pp. 1741-1744 (1998).

SUMMARY OF THE DISCLOSURE

In embodiments, there is provided a thin film transistor comprising:

(a) a semiconductor layer including a thiophene compound, wherein thethiophene compound comprises one or more substituted thiophene units,one or more unsubstituted thiophene units, and optionally one or moredivalent linkages;

(b) a gate dielectric; and

(c) a layer contacting the gate dielectric disposed between thesemiconductor layer and the gate dielectric, wherein the layer comprisesa substance comprising a fluorocarbon structure.

There is also provided in embodiments a thin film transistor comprising:

(a) a semiconductor layer including a thiophene compound, wherein thethiophene compound comprises one or more substituted thiophene units,one or more unsubstituted thiophene units, and optionally one or moredivalent linkages, wherein the thiophene compound is a polymer;

(b) a gate dielectric; and

(c) a layer contacting the gate dielectric disposed between thesemiconductor layer and the gate dielectric, wherein the layer comprisesa fluoroalkylsilane or a fluoroalkylphosphine, or a mixture thereof.

There is further provided in embodiments a process for fabricating athin film transistor comprising:

(a) forming a gate dielectric;

(b) forming a layer including a substance comprising a fluorocarbonstructure; and

(c) forming a semiconductor layer including a thiophene compoundcomprising one or more substituted thiophene units, one or moreunsubstituted thiophene units, and optionally one or more divalentlinkages,

wherein the layer contacts the gate dielectric and is disposed betweenthe semiconductor layer and the gate dielectric.

BRIEF DESCRIPTION OF THE DRAWINGS

Other aspects of the present invention will become apparent as thefollowing description proceeds and upon reference to the followingfigures which represent exemplary embodiments:

FIG. 1 represents a first embodiment of the present invention in theform of a thin film transistor;

FIG. 2 represents a second embodiment of the present invention in theform of a thin film transistor;

FIG. 3 represents a third embodiment of the present invention in theform of a thin film transistor; and

FIG. 4 represents a fourth embodiment of the present invention in theform of a thin film transistor.

Unless otherwise noted, the same reference numeral in different Figuresrefers to the same or similar feature.

DETAILED DESCRIPTION

A thin film transistor generally includes a gate electrode, a gatedielectric, a source electrode, a drain electrode, a semiconductor layerand an optional encapsulation layer. According to the presentdisclosure, the thin film transistor further contains a layer which isdisposed between the semiconductor layer and the gate dielectric,wherein the layer comprises a substance comprising a fluorocarbonstructure. In embodiments, the layer is a self-assembly monolayer(“SAM”). In other embodiments, there is present a plurality of two ormore layers (“multi-layers”) between the semiconductor layer and thegate dielectric, wherein each layer of the multi-layers comprises thesame or different substance with the same or different fluorocarbonstructure. In embodiments, the fluorocarbon structure is afluorine-containing polymer (having a degree of polymerization n of 2 ormore such as for example 2 to about 100). The SAM has a thickness forexample of less than about 5 nanometers, less than about 2 nanometers.Each layer of the multi-layers has a thickness for example of from about2 nanometers to about 500 nanometers, from about 5 nanometers to about100 nanometers. In embodiments, the layer is a crosslinked layer. Inembodiments, the substance is covalently bonded to the gate dielectricsurface. In other embodiments, the substance is not covalently bonded tothe gate dielectric surface. In embodiments, the layer is a SAMcomprising a substance covalently bonded to the gate dielectric surface.

In embodiments, the phrase “fluorocarbon structure” refers to an organiccompound/organic moiety analogous to hydrocarbons in which one or morehydrogen atoms has been replaced by fluorine. The fluorocarbon structurecan be a small molecule structure or a polymeric structure. Thefluorocarbon structure could be a linear or branched structure. Thefluorocarbon structure could be aliphatic, cyclic aliphatic, aromaticstructure, or mixture thereof. The phrase “fluorocarbon structure”encompasses “substituted fluorocarbon structure” and “unsubstitutedfluorocarbon structure.” In embodiments, the phrase “substitutedfluorocarbon structure” refers to replacement of one or more hydrogenatoms of the fluorine-containing organic compound/organic moiety withCl, Br, I and a heteroatom-containing group such as for example CN, NO₂,amino group (NH₂, NH), OH, COOH, alkoxyl group (O—CH₃), and the like,and mixtures thereof In embodiments, the phrase “unsubstitutedfluorocarbon structure” indicates that there is absent any replacementof a hydrogen atom of the fluorine-containing organic compound/organicmoiety with a substitutent described herein.

Unless otherwise noted, the phrase “fluorocarbon structure,” refers toboth the “unsubstituted fluorocarbon structure,” and the “substitutedfluorocarbon structure.” In embodiments, the fluorocarbon-containinglayer (e.g., SAM) is a product of a reaction of the gate dielectric anda precursor. The precursor comprises a material having the followingformula:X—Ywherein X is an reactive group with can react with certain functionalgroup(s) on the gate dielectric surface, and Y is a fluorocarbonstructure. In embodiments, X is selected from the groups of —PO₃H₃,—OPO₃H₃, —COOH, —SiCl₃, —SiCl(CH₃)₂, —SiCl₂CH₃, —Si(OCH₃)₃, —SiCl₃,—Si(OC₂H₅)₃, —OH, —SH, —CONHOH, —NCO, benzotriazolyl (—C₆H₄N₃), and thelike. The fluorocarbon structure in the fluorocarbon-containing layer isa fluorinated hydrocarbon comprising the following exemplary number ofcarbon atoms and fluorine atoms: 1 to about 60 carbon atoms, or fromabout 3 to about 30 carbon atoms; and 1 to about 120 fluorine atoms, orfrom 2 to about 60 fluorine atoms. In embodiments, the fluorocarbonstructure in the fluorocarbon-containing layer is a perfluorocarbonstructure. In embodiments, the carbon atoms of the fluorocarbonstructure in the fluorocarbon-containing layer are arranged in a chainof a length ranging for example from 3 to about 18 carbon atoms. Inembodiments, the fluorocarbon structure is a linear or branchedaliphatic or cyclic aliphatic group, a linear or branched groupcontaining aromatic groups and/or aliphatic or cyclic aliphatic group,or an aromatic group. Reaction of the X group with the gate dielectricsurface will result in a heteroatom containing moiety in the substance,wherein the heteroatom containing moiety is covalently bonded to boththe fluorocarbon structure and the gate dielectric. Such a “heteroatomcontaining moiety” is not to be confused with the “heteroatom-containinggroup” for the “substituted fluorocarbon structure.”

In embodiments, the precursor may be for example a fluoroalkylsilane ora fluoroalkylphosphine, or a mixture thereof, where the alkyl moietyincludes for instance 1 to about 50 carbon atoms.

In embodiments, the substance-containing layer which is disposed betweenthe gate dielectric and the semiconductor layer comprises a substancewhich includes a fluorine-containing polymer as the fluorocarbonstructure. In embodiments, the fluorine-containing polymer is alsocalled fluorinated polymer, which can be a fully or partiallyfluorinated polymer. The fluorine-containing polymer has a molecularweight for example from about 3000 to about 100,000. Any suitablefluorine-containing polymer may be used. Exemplary fluorine-containingpolymers may include for example fully fluorinated fluorocarbonplastics, including polytetrafluoroethylene and its fully fluorinatedcopolymers, and fluoroplastics, containing hydrogen or chlorine inaddition to fluorine on the carbon-carbon backbone. Specificfluorine-containing polymers may include for examplepolytetrafluoroethylene, polychlorotrifluoroethylene, poly(vinylfluoride), and poly(vinylidene fluoride).

In fabricating a thin film transistor having the substance-containinglayer, any suitable process may be employed. In embodiments where thesubstance in the layer is covalently bonded to the gate dielectric, thefollowing exemplary process is used. A deposition composition comprisinga precursor is deposited on the gate dielectric, wherein the precursorcomprises the fluorocarbon structure and a reactive group covalentlybonded to the fluorocarbon structure. Subsequent to deposition, thereactive group of the precursor reacts with the gate dielectric to formthe substance comprising the fluorocarbon structure and a heteroatomcontaining moiety covalently bonded to the fluorocarbon structure and tothe gate dielectric. In embodiments, the reactive group of the precursorspontaneously reacts with the gate dielectric.

In embodiments where the substance in the layer is not covalently bondedto the gate dielectric, for example a substance comprising afluorine-containing polymer, the following exemplary process is used. Adeposition composition comprising the substance in a suitable solvent isdeposited on the gate dielectric through a coating processing such asspin coating, dip coating, gravure, inkjet, microcontact, stencil, andstamping printing.

The deposition composition may be deposited on the gate dielectric byany suitable technique such as for example spin coating, dip coating,gravure, inkjet, microcontact, stencil, and stamping printing, and thelike.

In embodiments where drying of the deposited deposition composition isneeded, drying may occur at any suitable temperature such as roomtemperature or an elevated temperature above room temperature rangingfor example from about 30 to about 100 degrees C., where the drying timeranges for example from about 30 seconds to about 1 hour.

In embodiments, the deposition composition includes a liquid medium suchas for example toluene, chloroform, chlorobenzene, octane, heptane,hexane, alcohol, acetic acid, N,N-dimethylformamide, tetrahydrofuran,and the like. Where the deposition composition includes a liquid medium,the concentration of the precursor/substance ranges from about 10% toabout 90% by weight.

The thiophene compound present in the semiconductor layer may be apolymeric compound or a small molecule compound, or a mixture thereof.The phrase “polymeric compound” encompasses oligomers and polymers. Inembodiments, a mixture comprising two or more thiophene compounds may beused.

In embodiments, the substituted thiophene units have the formula:

wherein m is 1, 2 or 3,

wherein each substituted thiophene unit is the same or different fromeach other substituted thiophene unit in terms of substituent number,substituent identity, and substituent position, and

wherein each R₁ is independently selected from the group consisting of:

(a) a hydrocarbon group,

(b) a heteroatom containing group, and

(c) a halogen.

Any suitable numbers of the substituted thiophene units and theunsubstituted thiophene units may be present in the thiophene compound.The number of the substituted thiophene units ranges for example from atleast 1, or from 1 to about 30. The number of the unsubstitutedthiophene units ranges for example from at least 1, or from 1 to about30. These exemplary numbers of the substituted thiophene units and theunsubstituted thiophene units are for a small molecule thiophenecompound. In the embodiments where the thiophene compound is a polymericcompound, the total numbers of the substituted thiophene units and theunsubstituted thiophene units are a multiple of those disclosed hereindepending on the number of repeating units in the polymeric compound,where the number of repeating units may range for example from 2 toabout 100.

As part of the thiophene compound, the substituted thiophene unit(s) andthe unsubstituted thiophene unit(s) are covalent bonded at any availablering position(s) such as either or both of the second ring position andthe fifth ring position. The substituted thiophene unit(s) and theunsubstituted thiophene unit(s) may be disposed in any suitablearrangement with each other and with the divalent linkage(s).

Illustrative examples of the divalent linkage for the thiophene compoundare the following structures:

wherein n is 0, 1, 2, 3, or 4, and the substituents of R₄ are the sameor different from each other within each divalent linkage and amongdifferent divalent linkages. R₄ may be a hydrocarbon group, a heteroatomcontaining group, and a halogen.

Any suitable number of the same or different divalent linkage may bepresent in the thiophene compound such as for example from 1 to about 3divalent linkages. These exemplary numbers for the divalent linkage arefor a small molecule thiophene compound. In the embodiments where thethiophene compound is a polymeric compound, the total number of thedivalent linkage is a multiple of those disclosed herein depending onthe number of repeating units in the polymeric compound, where thenumber of repeating units may range for example from 2 to about 100.

Hydrocarbon Group for R₁, R₄

The hydrocarbon group contains for example from 1 to about 25 carbonatoms, or from 1 to about 10 carbon atoms, and may be for example astraight chain alkyl group, a branched alkyl group, a cycloalkyl group,an aryl group, an alkylaryl group, and an arylalkyl group. Exemplaryhydrocarbon groups include for example methyl, ethyl, propyl, butyl,pentyl, hexyl, heptyl, octyl, nonyl, decyl, undecyl, dodecyl, tridecyl,tetradecyl, pentadecyl, cyclopentyl, cyclohexyl, cycloheptyl, andisomers thereof.

The hydrocarbon group is optionally substituted one or more times withfor example a halogen (chlorine, bromine, fluorine, and iodine).

Heteroatom Containing Group for R₁, R₄

The heteroatom containing group has for example 2 to about 50 atoms, orfrom 2 to about 30 atoms, and may be for example a nitrogen containingmoiety, an alkoxy group, a heterocyclic system, an alkoxyaryl, and anarylalkoxy. Exemplary heteroatom containing groups include for examplecyano, nitro, methoxyl, ethoxyl, and propoxy.

Halogen for R₁, R₄

The halogen may be chlorine, bromine, fluorine, and iodine.

In embodiments, the substituent R₁ is regioregularly positioned on thethiophene compounds. The regioregularly positioned substituentsfacilitate proper alignment of the thiophene compound to form highlyordered microstructure domains in thin films. It is believed that thesethiophene compounds when deposited as thin films of, for example, about10 nanometers to about 500 nanometers form closely stacked lamellastructures that are conducive to efficient charge carrier transport. Theincorporated unsubstituted thiophene unit(s) and optional divalentlinkage(s) have some degree of rotational freedom, which helps todisrupt the extended π-conjugation of the thiophene compounds to anextent that may be sufficient to suppress its propensity towardsoxidative doping. Accordingly, in embodiments, these thiophene compoundsand the devices fabricated from these thiophene compounds are stable inambient conditions.

Any suitable small molecular thiophene compounds may be used in thesemiconductor layer such as those disclosed for example in Beng Ong etal., U.S. application Ser. No. 10/865,445 (Attorney Docket NumberA3055-US-NP), filed Jun. 10, 2004, the disclosure of which is totallyincorporated by reference.

In particular, exemplary small molecular thiophene compounds are forexample:

Any suitable polymeric thiophene compound may be used in thesemiconductor layer such as those disclosed for example in Ong et al.,U.S. Pat. No. 6,770,904 B2, the disclosure of which is totallyincorporated by reference.

Exemplary polymeric thiophene compounds are for example:

wherein for the polymeric thiophene compounds n is any suitable value,for example at least 2, or from 2 to about 100.

The thiophene compound may be a p-type semiconductor compound or an-type semiconductor compound, depending on the substituents. Ingeneral, substituents with electron-donating property such as ahydrocarbon like alkyl, alkyloxy and phenylene groups will make themolecule electron-rich, thus turning the molecule into p-type; whilesubstituents with electron-withdrawing ability such as cyano, nitro,fluoro, and fluorinated alkyl groups will make the thiophene moleculeelectron-deficient, thus turning the thiophene compound into a n-typesemiconductor.

Any suitable techniques may be used to form the semiconductor layercontaining the thiophene compound. One such method is by vacuumevaporation at a vacuum pressure of about 10⁻⁵ to 10⁻⁷ torr in a chambercontaining a substrate and a source vessel that holds the thiophenecompound in powdered form. Heat the vessel until the thiophene compoundsublimes onto the substrate. The performance of the films containing thethiophene compound depends on the rate of heating, the maximum sourcetemperature and/or substrate temperature during process. In embodiments,liquid deposition techniques may also be used to fabricate a thin filmcontaining the thiophene compound. The phrase “liquid depositiontechniques” refers to for example spin coating, blade coating, rodcoating, screen printing, ink jet printing, stamping and the like. Inembodiments, the thiophene compound is dissolved in a suitable liquid offor example tetrehydrofuran, dichlorormethane, chlororbenzene, toluene,and xylene at a concentration of about 0.1% to 10%, particularly 0.5% to5% by weight, followed by spin coating at a speed of about 500 to 3000rpm, particularly 1000-2000 rpm, for a period of time of about 5 to 100seconds, particularly about 30 to 60 seconds at room temperature or anelevated temperature.

In FIG. 1, there is schematically illustrated a thin film transistor(“TFT”) configuration 10 comprised of a substrate 16, in contacttherewith a metal contact 18 (gate electrode) and a layer of a gatedielectric 14, a layer 15 containing the substance comprising thefluorocarbon structure, on top of which two metal contacts, sourceelectrode 20 and drain electrode 22, are deposited. Over and between themetal contacts 20 and 22 is an organic semiconductor layer 12 asillustrated herein.

FIG. 2 schematically illustrates another TFT configuration 30 comprisedof a substrate 36, a gate electrode 38, a source electrode 40 and adrain electrode 42, a gate dielectric 34, a layer 35 containing thesubstance comprising the fluorocarbon structure, and an organicsemiconductor layer 32.

FIG. 3 schematically illustrates a further TFT configuration 50comprised of a heavily n-doped silicon wafer 56 which acts as both asubstrate and a gate electrode, a thermally grown silicon oxide gatedielectric 54, a layer 55 containing the substance comprising thefluorocarbon structure, and an organic semiconductor layer 52, on top ofwhich are deposited a source electrode 60 and a drain electrode 62.

FIG. 4 schematically illustrates an additional TFT configuration 70comprised of substrate 76, a gate electrode 78, a source electrode 80, adrain electrode 82, an organic semiconductor layer 72, a layer 73containing the substance comprising the fluorocarbon structure, and agate dielectric 74.

The substrate may be composed of for instance silicon, glass plate,plastic film or sheet. For structurally flexible devices, plasticsubstrate, such as for example polyester, polycarbonate, polyimidesheets and the like may be preferred. The thickness of the substrate maybe from about 10 micrometers to over 10 millimeters with an exemplarythickness being from about 50 to about 100 micrometers, especially for aflexible plastic substrate and from about 1 to about 10 millimeters fora rigid substrate such as glass or silicon.

The compositions of the gate electrode, the source electrode, and thedrain electrode are now discussed. The gate electrode can be a thinmetal film, a conducting polymer film, a conducting film made fromconducting ink or paste or the substrate itself, for example heavilydoped silicon. Examples of gate electrode materials include but are notrestricted to aluminum, gold, chromium, indium tin oxide, conductingpolymers such as polystyrene sulfonate-dopedpoly(3,4-ethylenedioxythiophene) (PSS-PEDOT), conducting ink/pastecomprised of carbon black/graphite or colloidal silver dispersion inpolymer binders, such as ELECTRODAG™ available from Acheson ColloidsCompany. The gate electrode can be prepared by vacuum evaporation,sputtering of metals or conductive metal oxides, coating from conductingpolymer solutions or conducting inks by spin coating, casting orprinting. The thickness of the gate electrode ranges for example fromabout 10 to about 200 nanometers for metal films and in the range ofabout 1 to about 10 micrometers for polymer conductors. The source anddrain electrodes can be fabricated from materials which provide a lowresistance ohmic contact to the semiconductor layer. Typical materialssuitable for use as source and drain electrodes include those of thegate electrode materials such as gold, nickel, aluminum, platinum,conducting polymers and conducting inks. Typical thicknesses of sourceand drain electrodes are about, for example, from about 40 nanometers toabout 1 micrometer with the more specific thickness being about 100 toabout 400 nanometers.

In embodiments, the gate dielectric is composed of one, two, or morelayers. The gate dielectric generally can be for example an inorganicmaterial film or an organic polymer film. Illustrative examples ofinorganic materials suitable as the gate dielectric include siliconoxide, silicon nitride, aluminum oxide, barium titanate, bariumzirconium titanate and the like; illustrative examples of organicpolymers for the gate dielectric include polyesters, polycarbonates,poly(vinyl phenol), polyimides, polystyrene, poly(methacrylate)s,poly(acrylate)s, epoxy resin and the like. The thickness of the gatedielectric is, for example from about 10 nanometers to about 500nanometers depending on the dielectric constant of the dielectricmaterial used. An exemplary thickness of the gate dielectric is fromabout 100 nanometers to about 500 nanometers. The gate dielectric mayhave a conductivity that is for example less than about 10⁻¹² S/cm.Exemplary gate dielectrics are disclosed in Janos Veres et al., “Gateinsulators in organic field-effect transistors,” Chem. Mater. Vol. 16,pp. 4543-4555 (published on web Sep. 11, 2004), the disclosure of whichis totally incorporated herein by reference.

In embodiments, the gate dielectric, the gate electrode, thesemiconductor layer, the source electrode, and the drain electrode areformed in any sequence where the gate electrode and the semiconductorlayer both contact the gate dielectric, and the source electrode and thedrain electrode both contact the semiconductor layer. The phrase “in anysequence” includes sequential and simultaneous formation. For example,the source electrode and the drain electrode can be formedsimultaneously or sequentially. The composition, fabrication, andoperation of field effect transistors are described in Bao et al., U.S.Pat. No. 6,107,117, the disclosure of which is totally incorporatedherein by reference.

The semiconductor layer has a thickness ranging for example from about10 nanometers to about 1 micrometer with a preferred thickness of fromabout 20 to about 200 nanometers. The TFT devices contain asemiconductor channel with a width W and length L. The semiconductorchannel width may be, for example, from about 1 micrometers to about 5millimeters, with a specific channel width being about 5 micrometers toabout 1 millimeter. The semiconductor channel length may be, forexample, from about 1 micrometer to about 1 millimeter with a morespecific channel length being from about 5 micrometers to about 100micrometers.

The source electrode is grounded and a bias voltage of generally, forexample, about 0 volt to about −80 volts is applied to the drainelectrode to collect the charge carriers transported across thesemiconductor channel when a voltage of generally about +20 volts toabout −80 volts is applied to the gate electrode.

Regarding electrical performance characteristics, a semiconductor layerof the present electronic device has a carrier mobility greater than forexample about 10⁻³ cm²/Vs (centimeters²/Volt-second) and a conductivityless than for example about 10⁻⁴ S/cm (Siemens/centimeter). The thinfilm transistors produced by the present process have an on/off ratiogreater than for example about 10³ at 20 degrees C. The phrase on/offratio refers to the ratio of the source-drain current when thetransistor is on to the source-drain current when the transistor is off.

The invention will now be described in detail with respect to specificexemplary embodiments thereof, it being understood that these examplesare intended to be illustrative only and the invention is not intendedto be limited to the materials, conditions, or process parametersrecited herein. All percentages and parts are by weight unless otherwiseindicated. As used herein, room temperature refers to a temperatureranging for example from about 20 to about 25 degrees C.

COMPARATIVE EXAMPLE 1

There was selected a top-contact thin film transistor configuration asschematically illustrated, for example, in FIG. 3.

The device was comprised of a n-doped silicon wafer with a thermallygrown silicon oxide layer of a thickness of about 110 nanometersthereon. The wafer functioned as the gate electrode while the siliconoxide layer acted as the gate dielectric and had a capacitance of about30 nF/cm² (nanofarads/square centimeter) as measured with a capacitormeter. The fabrication of the device was accomplished at ambientconditions without any precautions to exclude the materials and devicefrom exposure to ambient oxygen, moisture, or light. The silicon waferwas first cleaned with isopropanol, air dried, then cleaned with anargon plasma and washed with distilled water and isopropanol. Thissurface has an advanced water contact angle of 37±2°.

The following polythiophene (having a structure as previously shown inembodiment (e)) was used to fabricate the semiconductor layer, wherethis polymer possessed a M_(w) of 22,900 and M_(n) of 17,300 relative topolystyrene standards. This polythiophene and its preparation aredescribed in U.S. Patent Application Publication No. 2003/0160230, thedisclosure of which is totally incorporated herein by reference. Thesemiconductor polythiophene layer of about 30 nanometers in thicknesswas deposited on top of the device by spin coating of the polythiophenein dichlorobenzene solution at a speed of 1,000 rpm for about 100 toabout 120 seconds, and dried in vacuo at 80° C. for about 2 to about 10hours. Thereafter, the gold source and drain electrodes were depositedon top of the semiconductor polythiophene layer by vacuum depositionthrough a shadow mask with various channel lengths and widths, thuscreating a series of transistors of various dimensions.

The evaluation of field-effect transistor performance was accomplishedin a black box at ambient conditions using a Keithley 4200 SCSsemiconductor characterization system. The carrier mobility, μ, wascalculated from the data in the saturated regime (gate voltage,V_(G)<source-drain voltage, V_(SD)) according to equation (1)I _(SD) =C _(i)μ(W/2L)(V _(G) −V _(T))²  (1)where I_(SD) is the drain current at the saturated regime, W and L are,respectively, the semiconductor channel width and length, Ci is thecapacitance per unit area of the gate dielectric layer, and V_(G) andV_(T) are, respectively, the gate voltage and threshold voltage. V_(T)of the device was determined from the relationship between the squareroot of I_(SD) at the saturated regime and V_(G) of the device byextrapolating the measured data to I_(SD)=0.

Another property of the field-effect transistor is its current on/offratio. This is the ratio of the saturation source-drain current when thegate voltage V_(G) is equal to or greater than the drain voltage V_(D)to the source-drain current when the gate voltage V_(G) is zero.

Thin film transistors with channel lengths of about 90 micron andchannel widths of about 5000 microns were characterized by measuring theoutput and transfer curves. The devices exhibited field-effect mobilityof 0.004 cm²/V·s and current on/off ratio of about 10⁴-10⁵.

COMPARATIVE EXAMPLE 2

In this comparative example, the devices were fabricated using the sameprocedure in Comparative Example 1 except that the gate dielectricsurface was modified with a layer of octyltrichlorosilane (OTS8) asfollows. The cleaned substrates were immersed in a 0.1 M solution ofoctyltrichlorosilane in toluene for about 20 minutes at about 60° C. Thesubstrates were subsequently washed with toluene and isopropanol anddried before deposition of the semiconductor layer. The formation of theOTS8 layer was verified by contact angle measurement. The surface has anadvanced water contact angle of 91±1°.

Thin film transistors with channel lengths of about 90 micron andchannel widths of about 5000 microns were characterized by measuring theoutput and transfer curves. The devices exhibited field-effect mobilityof 0.01-0.038 cm²/V·s and current on/off ratio of about 10⁶-10⁷.

EXAMPLE 1

In this example, the devices were fabricated using the same procedure inComparative Example 1 except that the gate dielectric surface wasmodified with a layer of 1H,1H,2H,2H-perfluorooctyltrichlorosilane(FOTS8) as follows. The cleaned substrates were immersed in a 0.01 Msolution of FOTS8 in toluene for about 20 minutes at about 60° C. Thesubstrates were subsequently washed with toluene and isopropanol anddried before deposition of the semiconductor layer. The formation of theFOTS8 layer was verified by contact angle measurement. The surface hasan advanced water contact angle of 103±2°.

Thin film transistors with channel lengths of about 90 micron andchannel widths of about 5000 microns were characterized by measuring theoutput and transfer curves. The devices exhibited field-effect mobilityof 0.06-0.1 cm²/V·s and current on/off ratio of about 10⁶-10⁷. Themobility was improved by 150 to 250 times compared with the deviceswithout a modification layer (Comparative Example 1), and improved by 3to 5 times compared with the device modified with a layer of OTS8(Comparative Example 2).

It will be appreciated that various of the above-disclosed and otherfeatures and functions, or alternatives thereof, may be desirablycombined into many other different systems or applications. Variouspresently unforeseen or unanticipated alternatives, modifications,variations or improvements therein may be subsequently made by thoseskilled in the art which are also intended to be encompassed by thefollowing claims. Unless specifically recited in a claim, steps orcomponents of claims should not be implied or imported from thespecification or any other claims as to any particular order, number,position, size, shape, angle, color, or material.

1. A process for fabricating a thin film transistor comprising: (a)forming a gate dielectric; (b) forming a layer including a substancecomprising a fluorocarbon structure; and (c) forming a semiconductorlayer including a thiophene compound comprising one or more substitutedthiophene units, one or more unsubstituted thiophene units, andoptionally one or more divalent linkages, wherein the layer contacts thegate dielectric and is disposed between the semiconductor layer and thegate dielectric.
 2. The process of claim 1, wherein the forming thesubstance-containing layer comprises depositing a deposition compositioncomprising the substance.
 3. The process of claim 1, wherein the gatedielectric comprises silicon oxide.
 4. The process of claim 1, whereinthe layer is a self-assembled monolayer.
 5. The process of claim 1,wherein the layer has a thickness ranging from about 0.5 nm to about 500nm.
 6. The process of claim 1, wherein the forming thesubstance-containing layer comprises depositing a deposition compositioncomprising a precursor, wherein the precursor comprises the fluorocarbonstructure and a reactive group covalently bonded to the fluorocarbonstructure, wherein the reactive group reacts with the gate dielectric toform the substance comprising the fluorocarbon structure and aheteroatom containing moiety covalently bonded to the fluorocarbonstructure and to the gate dielectric.
 7. The process of claim 6, whereinthe heteroatom containing moiety comprises a silicon atom or aphosphorus atom.
 8. The process of claim 1, wherein the substance is afluoroalkylsilane.
 9. The process of claim 1, wherein the fluorocarbonstructure is a perfluorocarbon structure.
 10. The process of claim 1,wherein the thiophene compound is a polymeric compound.
 11. The processof claim 1, wherein the thiophene compound is a homopolymer.
 12. Theprocess of claim 1, wherein the thiophene compound is a small moleculecompound.
 13. The process of claim 1, wherein the thiophene compoundexhibits a regioregular structure.
 14. The process of claim 1, whereinthe fluorocarbon structure is an unsubstituted fluorocarbon structure.